In recent years, a contactless power-feeding system for supplying power to devices without power cables is in practical use. In the contactless power-feeding system, a PC (Personal Computer) and a POS (Point of Sales) terminal are located on a power-feeding device. The contactless power-feeding system contactlessly supplies power to the PC and the POS terminal through the power-feeding device. Further, the contactless power-feeding system contactlessly supplies power to power-receiving devices of peripheral devices which include a printer device and the like.
When a coil of a power-feeding device is located close to a coil of a power-receiving device, the resonance phenomenon of the magnetic field is generated between the coils. In the contactless power-feeding system, a power-feeding device supplies power to a power-receiving device by using the resonance phenomenon, for example.
In the contactless power-feeding system, in order to make the power-transmission efficiency stable, the output voltage of a rectifier circuit of the power-receiving device is fed back to the power-feeding device. The contactless power-feeding system is configured to thereby control the voltage input in the power-feeding device side.
However, the current output from an inverter circuit of the power-feeding device and the discharge current of a battery of the power-receiving device may change rapidly depending on the variation of the gap between the primary coil of the power-feeding side and the secondary coil of the power-receiving side and depending on the variation of a load connected to the power-receiving side. When the current changes rapidly as described above, over-discharge and over-charge may occur in the contactless power-feeding system. In some cases, the contactless power-feeding system may stop operation, i.e., temporary blackout may occur.